This paper reports an XPS surface study of pure phase BiFeO${}_3$ thin film produced and later etched by pure argon ions. Analysis of high-resolution spectra from Fe $2p$, Bi $4f$ and $5d$, O $1s$, and the valence band, exhibited mainly Fe${}^{3+}$ and Bi${}^{3+}$ components, but also reveal Fe${}^{2+}$. High-energy argon etching induces the growth of Fe${}^{\left(0\right)}$ and Bi${}^{\left(0\right)}$ and an increment of Fe${}^{2+}$, as expected. The BiFeO${}_3$ semiconductor character is preserved despite the oxygen loss, an interesting aspect for the study of the photovoltaic effect through oxygen vacancies in some ceramic films. The metal-oxygen bonds in O $1s$ spectra are related only to one binding energy contrary to the split from bismuth and iron reported in other works. All these data evidence that the low-pressure argon atmosphere is proved to be efficient to produce pure phase BiFeO${}_3$, even after argon etching.

Fe₂O₃ nanoparticles with sizes ranging from 15 to 53 nm were synthesized by a modified sol-gel method. Maghemite particles as well as particles with admixture of maghemite and hematite were obtained and characterized by XRD, FTIR, UV-Vis photoacoustic and Mössbauer spectroscopy, TEM, and magnetic measurements. The size and hematite/maghemite ratio of the nanoparticles were controlled by changing the Fe:PVA (poly (vinyl alcohol)) monomeric unit ratio used in the medium reaction (1:6, 1:12, 1:18, and 1:24). The average size of the nanoparticles decreases, and the maghemite content increases with increasing PVA amount until 1:18 ratio. The maghemite and hematite nanoparticles showed cubic and hexagonal morphology, respectively. Direct band gap energy were 1.77 and 1.91 eV for A6 and A18 samples. Zero-field-cooling–field-cooling curves show that samples present superparamagnetic behavior. Maghemite-hematite phase transition and hematite Néel transition were observed near 700 K and 1015 K, respectively. Magnetization of the particles increases consistently with the increase in the amount of PVA used in the synthesis. Mössbauer spectra were adjusted with a hematite sextet and maghemite distribution for A6, A12, and A24 and with maghemite distribution for A18, in agreement with XRD results.

Fe₂O₃ nanoparticles (as maghemite and hematite mixtures) were prepared using adapted sol-gel method from a polyvinylpyrrolidone (PVP) aqueous solution in various Fe³⁺ : PVP monomer ratios. Analysis of X-ray diffraction was obtained to evaluate the crystalline state, average crystallite size, and composition of iron oxide phases. The morphology and average nanoparticles size were evaluated by electronic transmission microscopy. Magnetic properties of iron oxide were analyzed at low and room temperatures. Optical characteristics were evaluated by UV-Vis photoacoustic spectroscopy and the Mössbauer spectrum was obtained in order to evaluate subtle changes in the nuclear environment of the iron atoms.

We present a study of the substitution of Mn by Cu on the compound $\mathrm{N}{\mathrm{i}}_2$ $\mathrm{M}{\mathrm{n}}_{1-x}$ $\mathrm{C}{\mathrm{u}}_x$ $\mathrm{G}{\mathrm{a}}_{0.9}$ $\mathrm{A}{\mathrm{l}}_{0.1}$ , showing that the substitution of a small amount of Al on the Ga site does not affect the magnetic and magnetocaloric potential compared to $\mathrm{N}{\mathrm{i}}_2$ (Mn,Cu)Ga alloy. The samples were prepared with 10% substitution of Al and with Cu concentrations of x = 0.0, 0.2, and 0.3. Magnetization measurements as a function of temperature performed from 10 to 400 K, with an applied field of 0.02 T showed a ferromagnetic state, with critical temperature $T_c$ = 295 and 300 K for the samples with Cu, x = 0.2 and 0.3, respectively. For the sample without Cu, a complex behavior is observed at $T_c$ = 370 K, with martensitic transition at 220 K and a premartensitic at 250 K. Analysis of x-rays diffractograms at room temperature show a L $2_1$ structure for x = 0.0, while for x = 0.2 a mixture of L $2_1$ and martensitic is present, and the sample with x = 0.3 it is in a fully martensitic phase. Heat capacity measurements were performed in order to calculate magnetocaloric effect in the samples. The results indicate that in Ni(Mn,Cu)Ga alloys, a partial substitution of Ga by Al still produce a high refrigerant capacity while reducing the costs of fabrication.